Those skilled in the art of manufacturing and designing industrial gaskets have historically been skilled in the manufacturing practice of compressing asbestos fiber sheets that are later cut and shaped into industrial gaskets. Typically, compressed asbestos sheets are formed by adding a rubber binder to a base of asbestos fibers. A two roller sheeter machine then converts the resultant mixture into sheet form. Finally, the resultant mixture is exposed to high pressure and temperature.
Initially, compressed sheets made from asbestos were standard for forming gaskets due to the superior temperature properties of asbestos. Despite the utility of compressed asbestos sheets, their use has been severely curtailed, especially in environments where there is a potential for human ingestion of asbestos, since it was discovered that asbestos can cause serious health problems.
As a result, numerous attempts have been made to manufacture compressed sheets for gasketing applications without using asbestos. However, none of these alternative industrial fibers have been able to withstand the high temperatures that asbestos gaskets can.
Obviously, the goal has been to manufacture a compressed non-asbestos sheet that retains the properties of the compressed asbestos sheets they replaced. Typically, these replacements for asbestos sheets have randomly distributed fibers bonded together with as much as twenty-five (25) percent of a rubber elastomer which cannot withstand temperatures above 500.degree. Fahrenheit. For example, U.S. Pat. No. 4,859,526 to Potepan et al. discloses a compressed non-asbestos sheet made with a fiber base composed of carbon fibers mixed with a small proportion of organic fibers such as aramid fibers. The carbon fibers have a degree of carbonization greater than ninety (90) percent and a modulus of elasticity below 10,000,000 psi. An elastomeric material is used to bind the carbon fibers and organic fibers. These rubber bonded sheets include organic fibers, rubber binders, fillers and curatives in their composition.
Because existing non-asbestos compressed sheets, such as those described in Potepan, are typically bonded with an elastomeric rubber material, gaskets formed from such sheets cannot withstand high operating temperatures. Elastomeric rubber materials typically have operating temperature limits ranging from 250.degree. Fahrenheit to 450.degree. Fahrenheit. Similarly, the aramid fibers used to form such sheets have an upper temperature limit of 550.degree. Fahrenheit. Thus, the elastomeric rubber materials and aramid fibers have temperature limitations well below the typical temperatures which compressed sheet gaskets made from such sheeting are frequently exposed to. Despite these limitations, gaskets composed of existing non-asbestos compressed sheets containing such elements are operated at temperatures well above the individual temperature limits of the materials because those materials carbonize without excessive loss of volume. However, when this happens, the gaskets lose their resiliency and strength.
As previously discussed, existing non-asbestos sheets often contain in excess of twenty (20) percent rubber used as a binder. Because rubber tends to oxidize into a powder between hot flanges, the rubber will lose its cohesion properties, thereby making existing non-asbestos compressed sheets unsuitable for use at high temperatures.
Another problem with existing non-asbestos compressed sheets is that the properties of the sheeting diminish significantly during storage. Typically, this sheeting shelf-cures during storage causing the sheeting to harden or age. After only a relatively short period of time, the sheeting becomes, for all practical purposes, unusable.
Existing compressed sheets are not suitable for many industrial applications, such as gasketing in nuclear power plant reactor coolant systems. The curatives, fillers, additives and sulfur based compounds used to manufacture existing compressed sheets may be released in the presence of treated reactor coolant to form corrosive chemical contaminants. Those chemical contaminants leach in to the treated coolant and can attack vital metal parts possibly resulting in dire consequences.
Finally, the process for making existing non-asbestos compressed sheets also involves the use of solvents. The use of solvents poses a substantial health risk to the people manufacturing the compressed sheets. Further, the use of solvents requires expensive solvent recovery systems that significantly increase the manufacturing cost of the compressed sheets.
Consequently, gaskets made from compressed non-asbestos sheets, such as those made in accordance with Potepan, have relatively low high temperature limits, shorter life spans, and require more maintenance than the asbestos gaskets they replaced. Compressed sheet gaskets made from other asbestos substitutes suffer from similar deficiencies.
A need, therefore, exists for a non-asbestos compressed sheet A that can be manufactured into a gasket which can perform as well as or better than gaskets made from asbestos sheeting without the health consequences incident to asbestos.
Gaskets made from existing non-asbestos compressed sheets are solid in nature and tend to break when bent or folded. A need, therefore, also exists for a non-asbestos compressed sheet that has excellent workability and at the same time retain its integrity.
While cables of braided graphite fiber theoretically have been known for years, their incorporation into compressed sheets has been largely unsuccessful because of the elusiveness of effective reinforcement of vermiculated natural flake graphite. An effective method of joining a ribbon of graphite foil to reinforcing strands while still maintaining its extreme service temperature capability has not been possible until now. Consequently, until now, it was not possible to create gasketing sheets made from metallic and carbon fiber reinforced graphite yarns that can withstand service temperatures ranging from about negative 400.degree. Fahrenheit to about 5400.degree. Fahrenheit.
A significant improvement in non-asbestos sheets is disclosed in U.S. Pat. No. 5,683,778 to Crosier which discloses a fiber reinforced composite strand having a reinforcing fiber core, an adhesive a vermiculated graphite jacket. While having much utility, the disclosed strand has insufficient shear resistance for many gasketing applications requiring flattened sheets.